If you are a woman, the odds are pretty good that you have experienced the agony and annoyance of a urinary tract infection at some point in your life. And if you are a particularly unlucky woman, you might experience them on the regs. While women often swear sex is the culprit of a UTI showing up again, as far as science is concerned, exactly why these recurring infections happen has been a mystery.

New research from Washington University published this week in the journal PLOS Pathogens sheds some new light on why getting jiggy with it can, in fact, result in recurring UTIs.

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In young, sexually active women, somewhere around 80 percent of UTIs are caused by the bacteria E.coli, usually when fecal particles reach the urethra, which connects the bladder to the outside of the body. Sex is a pretty good way for E.coli to get in there in the first place—to put it delicately, all that thrusting shoves a fair amount of bacteria up your hoo-ha. Common wisdom has held that recurring UTIs are simply caused by new bacteria being introduced into the urinary tract. But that doesn’t explain some common aspects of recurring infections, such as the fact that every time a woman gets an infection, she becomes more likely to get one again.

Working in mice, Washington University researchers found that the culprit may in fact be a different bacteria: Gardnerella vaginalis. G. vaginalis is a vaginal bacteria that can cause bacterial vaginosis. Like other bacteria, sex can push it up into the urinary tract. The researchers found that while it did not cause infection directly, it damaged cells on the surface of the bladder, allowing E.coli from a previous UTI to come out of hiding and start multiplying again, leading to another infection. In some of the mice, the bacteria traveled on to the kidneys, causing damage there.

Here, behold the face of the bacteria you should blame:

Image: Washington University

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The researchers noted that G. vaginalis is probably not the only cause of recurring UTIs—some of them likely are from newly-introduced E.coli. But the research suggests that when G. vaginalis shows up in a UTI lab test, doctors should consider whether it’s the real source of a problem. This is key, because the antibiotics that work on standard UTIs are different from the antibiotics used to treat G. vaginalis. And at a time when antibiotic resistant E. coli have made UTIs virtually untreatable in some places, it points to an alternate route to dealing with these nasty infections.